1. Technical Field of the Invention
The present invention relates to novel improvedly photostable sunscreen compositions comprising at least one UV-photoprotecting compound and a photostabilizing effective amount of at least one silane or organosiloxane compound bearing a 2-hydroxybenzophenone substituent.
2. Description of the Prior Art
It is known to this art that light radiation with wavelengths of from 280 nm to 400 nm promotes tanning of the human epidermis, and that light rays with wavelengths more particularly from 280 to 320 nm, i.e., UV-B irradiation, cause skin burns and erythema which may be harmful to the natural development of the tan. For these reasons, as well as for aesthetic reasons, there is constant demand for means of controlling this natural tanning in order thereby to control the color of the skin; this UV-B radiation should thus be screened from the skin.
It is also known to this art that UV-A radiation, with wavelengths of from 320 to 400 nm, which causes tanning of the skin, also adversely affects it, especially in the case of sensitive skin or of skin which is continually exposed to solar radiation. UV-A rays cause, in particular, a loss of elasticity of the skin and the appearance of wrinkles, leading to premature skin aging. Such irradiation promotes triggering of the erythemal reaction or amplifys this reaction in certain individuals and may even be the source of phototoxic or photoallergic reactions. Thus, for aesthetic and cosmetic reasons, such as conservation of the natural elasticity of the skin, for example, an ever-increasing number of individuals wish to control the effect of UV-A rays on their skin. It is thus desirable to also screen out UV-A radiation.
In this respect, a particularly advantageous class of UV-A screening agents currently includes dibenzoylmethane derivatives, and in particular 4-tert-butyl-4'-methoxydibenzoylmethane, which effectively have a high intrinsic power of absorption. These dibenzoylmethane derivatives, which are compounds well known per se as screening agents that are active in the UV-A range, are particularly described in FR-A-2,326,405 and FR-A-2,440,933, as well as in EP-A-0,114,607; 4-(tert-butyl)-4'-methoxydibenzoylmethane is moreover currently commercially available under the trademark "Parsol 1789" from Givaudan.
Unfortunately, it too is known that dibenzoylmethane compounds are relatively sensitive to ultraviolet radiation (especially UV-A), i.e., more precisely, they are known to have an annoying tendency to become degraded more or less quickly under the action or influence of this radiation. Thus, this substantial lack of photochemical stability of dibenzoylmethane derivatives with respect to ultraviolet radiation, to which they are by nature intended to be subjected, does not guarantee constant protection during prolonged exposure to the sun, such that, in a restrictive manner, repeated applications at regular, close intervals of time must be carried out by the user in order to obtain effective protection of the skin against UV rays.
Too, EP-A-0,709,080 proposes combining benzalmalonate derivatives with the dibenzoylmethane derivatives in order to reduce the photoinstability of said dibenzoylmethane compounds. However, the photostabilization of the dibenzoylmethane derivatives with respect to UV radiation to date remains a vexing problem which still not been solved entirely satisfactorily.
It has also been proposed, in EP-A-0,843,996, to formulate 2-hydroxybenzophenone hydrocarbon-based derivatives such as 2-hydroxy-4-methoxybenzophenone or 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid with the dibenzoylmethane compounds in order to reduce the photoinstability of said dibenzoylmethane compounds.
Another difficulty, independent of that mentioned above, encountered with dibenzoylmethane compounds is that these are lipophilic UV-screening agents which also present the drawback of being solid at room temperature. Consequently, including same in sunscreen cosmetic compositions entails certain constraints as regards their formulation and practical applications thereof in particular in respect of determining solvents for properly dissolving same, whether alone or together with other screening agents. In this regard, oils such as esters are conventionally employed, more particularly C.sub.12 -C.sub.15 alkyl benzoates ("Finsolv TN" marketed by Finetex), or triglycerides and especially C.sub.8 -C.sub.12 fatty acid triglycerides ("Miglyol 812" marketed by Huls), but these various materials exhibit solubilizing properties vis-a-vis the abovementioned screening agents which still remain insufficient.
The sunscreen/antisun formulations based on dibenzoylmethane compounds and 2-hydroxy-4-methoxybenzophenone or 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid, as described in EP-A-0,843,996, do not satisfactorily solve this problem of solubility of said dibenzoylmethane derivatives.